Traveling wave electrooptic devices, in which the optical wave and the modulating (i.e., microwave) electrical wave propagate with the same velocities, have very broad operating bandwidths. However, for important materials used in the fabrication of these devices, such as lithium niobate, there is an inherent mismatch between the two velocities. As a consequence if modulation above the "walk-off" frequency is to be obtained, the electrical signal wavepath must be specially designed in order to compensate for the velocity mismatch which exists. In the traveling wave devices disclosed in the copending application of P. P. Liu, Ser. No. 318,353, filed Nov. 5, 1981, now U.S. Pat. No. 4,468,086, the problem of velocity mismatch is dealt with by means of a meandering electrode. The electrode is shaped so as to interact with the optical wavepath over a first interval where the direction of the electrical signal has one sense (i.e., polarity), and not to interact over a second interval where the sense is reversed. In the traveling wave device disclosed in the copending application of R. C. Alferness, Ser. No. 321,475, filed Nov. 16, 1981, now U.S. Pat. No. 4,448,479, there is interaction over the entire length of the electrode. However, the electrodes are periodically displaced so as to introduce a structural polarity reversal which compensates for the electrical polarity reversal caused by the velocity mismatch.
In both of the above-described types of devices the resulting frequency characteristic includes a single peak at a designated frequency. Furthermore, the ability to control where this bandwidth falls (i.e., what frequencies are included within the response characteristic of the device) is limited. What is needed is a technique for distributing the available bandwidth over the frequency spectrum of interest. For example, to generate or switch a pulse train of very narrow pulses requires a device whose response includes a plurality of harmonically related passbands. Prior art traveling wave devices do not possess such a frequency characteristic. It is, accordingly, the broad object of the present invention to provide a mechanism for both varying and increasing the frequency characteristic of traveling wave electrooptic devices.